The present invention is directed to the manufacture of a bent portion of a pipeline and, in particular to a pipeline having a large cross-sectional measurement. Pipelines of this type are used in civil engineering or public works, for example, for the construction of thermal or nuclear power stations.
The manufacture of curved pipes or bends, that is, the manufacture of hollow toroidal segments of circular cross-section and of various thicknesses which are used together with cylindrical pipelines presents difficulties, especially if the constituent material is a metal and if the environmental stresses to which they are subjected, such as pressure, fatigue, and corrosion are high.
Several techniques are currently used for manufacturing these curved pipes. For example, these curved pipes may be formed using traditional molding processes, namely sand molding or chill casting, with a core. Alternatively, these curved pipes may be formed by mechanical bending of tube elements under the action of heat or in the cold, by bending using electrical induction, by hot deformation by the forging of tubular pieces, or by welding of stamped shells.
The continuity of the toroidal wall is sometimes sacrificed by welding together sections of welded or seamless tube elements, to the detriment of the quality of the flow of the fluid to be conveyed therethrough.
Apart from traditional molding, whose technical limits result from the severity of the specifications, the other methods consist of deforming the constituent material of a tube. This leads, in the majority of cases, to a lack of homogeneity in the thickness of the walls and also to damage to the original structure of the material. Alternatively, this introduces numerous welds into a portion of piping which is already particularly sensitive to erosion, pitting, corrosion and so on, because of the deflection of the stream of fluid running through it.
The present invention provides a method which overcomes the above recited disadvantages by combining the techniques of centrifugation of materials in the liquid phase and the techniques of machining these same materials in the solid phase.
Furthermore, the present invention makes it possible to produce very thick curved pipes capable of withstanding high stresses without damage to the structure obtained during the initial shaping, except the stresses resulting from the heat treatments frequently applied to materials and, in particular, to metal materials, after solidification.
It is known that centrifugation under high acceleration provides the materials produced in this way with good characteristics and an exceptional internal constitution.
In fact, the density differences between these materials in the liquid state and the impurities which could be present therein in the form of inclusions are considerably increased by the centrifugal force. The impurities are thus thrown out, either towards the bore in the centrifuged hollow body, under the influence of the Archimedean thrust, or towards the periphery, under the influence of the centrifugal force, depending on whether they are more or less dense than the material in question.
The dissolved gases are also excluded from the liquid mass under the influence of the pressure difference between the ambient environment, generally the atmosphere and the body of the liquid subjected to the centrifugal force before and during its solidification.
For the same reasons, the centrifuged materials also possess no pores.
If the processing and the centrifugation of these materials are carried out correctly, they are capable of withstanding high pressures because of their noteworthy water tightness.
A centrifuged body can be given any desired external shape, provided that this shape enables it to be released from the mold, which is virtually always made of metal, either directly or by splitting the mold.
However, the bore in this body will theoretically be either a cylinder or revolution, if the centrifugation axis is horizontal, or a portion of a paraboloid of revolution, if the centrifugation axis is vertical. The axis of symmetry of both these surfaces is identical to the axis of rotation of the mold. The paraboloid of revolution is closer to a cylinder of revolution, as the centrifugal force increases relative to the earth's gravity.
The internal and external surfaces of a circular cross-section are two curved pipe toroids with the same center and the same mean radius. The thickness to be obtained is equal to the difference in radius of the concentric circular sections of the external surface and the internal surface.
The novel method and apparatus according to the present invention makes it possible to obtain the external surface of the toroid by centrifugation, while the internal surface will be obtained by machining the crude surface, of virtually cylindrical shape, resulting from the centrifugal force.
As the principle is the same for both horizontal and vertical centrifugations, the application of the invention to the vertical centrifugation of a curved pipe bend will be described below. To simplify the account, it will be assumed that the centrifugation mold is made of metal, although it is understood that it could consist of any other appropriate material.